IoC ConfigUser Guide


IoC Config is an IoC library for providing injectable config values.

Config values are essentially constants, but their value may be overridden on registry startup.

This makes them great for use by 3rd party libraries. The libraries can set sensible default values, and applications may then optionally override them.

IoC Config also grabs values from environment variables and .props files. It even provides environmental overrides allowing your test server to run with different configuration than your dev, or prod server!

Quick Start

  1. Create a text file called
    using afIoc
    using afIocConfig
    class Example {
        @Config { id="my.config" }
        Str? myConfig
    const class OtherModule {
        @Contribute { serviceType=FactoryDefaults# }
        Void contributeFactoryDefaults(Configuration config) {
            config["my.config"] = "3rd party libraries set Factory defaults"
    const class AppModule {
        @Contribute { serviceType=ApplicationDefaults# }
        Void contributeApplicationDefaults(Configuration config) {
            config["my.config"] = "Applications override Factory defaults"
    class Main {
        Void main() {
            registry := RegistryBuilder()
            example  := (Example)
            echo("--> ${example.myConfig}")  // --> Applications override Factory defaults
  2. Run as a Fantom script from the command line:
    C:\> fan
    [afIoc] Adding module definitions from pod 'afIocConfig'
    [afIoc] Adding module afIocConfig::IocConfigModule
    [afIoc] Adding module Example_0::AppModule
    [afIoc] Adding module Example_0::OtherModule
       ___    __                 _____        _
      / _ |  / /_____  _____    / ___/__  ___/ /_________  __ __
     / _  | / // / -_|/ _  /===/ __// _ \/ _/ __/ _  / __|/ // /
    /_/ |_|/_//_/\__|/_//_/   /_/   \_,_/__/\__/____/_/   \_, /
                                Alien-Factory IoC v3.0.0 /___/
    IoC Registry built in 281ms and started up in 18ms
    --> Applications override Factory defaults
    [afIoc] IoC shutdown in 7ms
    [afIoc] IoC says, "Goodbye!"

Define Config

All config values are referenced by a unique config id (a string). This id is used to set factory default values, application values, and to inject the value in to a service.

Start by contributing to the FactoryDefaults service in your AppModule to set a default value:

@Contribute { serviceType=FactoryDefaults# }
Void contributeFactoryDefaults(Configuration config) {
    config["configId"] = "README.MD"

Config's may take any value as long as it is immutable (think const class).

Anyone may then override your value by contributing to the ApplicationDefaults service:

@Contribute { serviceType=ApplicationDefaults# }
Void contributeApplicationDefaults(Configuration config) {
    config["configId"] = "readme.txt"

Inject Config

Use the @Config facet to inject config values into your service.

class MyService {
    @Config { id="configId" }
    File configValue


Note that when config values are injected, they are Type coerced to the field type. That means you can contribute Str or Uri values and inject them as a File.

If @Config does not supply an id then it is determined from the field name, class name and pod name. For example, if Type MyService was in a pod called acme and looked like:

class MyService {
    File configValue


Then the following IDs would be looked up (in order):

<field>                --> configValue
<pod>.<field>          --> acme.configValue
<pod>.<class>.<field>  --> acme.MyService.configValue
<class>.<field>        --> MyService.configValue

(Note that config IDs are case insensitive.)

If the config value is still not found then, as a last resort, the field name is checked against the config IDs after they have been stripped of any non-alphaNum characters. That means you can inject config values with IDs similar to afIocEnv.isDev with:

@Config Bool afIocEnvIsDev

Default Config

Config values are automaticially added from:

  • environment variables
  • a config.props file
  • a secret.props file

config.props and secret.props should be in the current directory where Fantom was started, and is a handy starting place for externalising config.

Config is added in the following order:

  • afIocConfig.factoryDefaults
  • afIocConfig.envVars
  • afIocConfig.configFile
  • afIocConfig.secretFile
  • afIocConfig.applicationDefaults

With later contributions overriding previous ones.

It is conventional to place standard application config in config.props and any user or environment specific overrides in secret.props. config.props is then checked into Source Code Management, but secret.props is usually kept local to the executing machine.

Custom Config

To add your own sources of config, contribute ConfigProvider instances to ConfigSource. ConfigProvider has helper methods for creating instances from Str maps and .props files.

@Contribute { serviceType=ConfigSource# }
Void contributeConfigSource(Configuration config) {
    config["myApp.config"] = ConfigProvider(`app.props`.toFile)

Environment Overrides

Let's say you define a database connection URL:

jdbcUrl = jdbc:mysql://localhost:3306/fanlocal

While this may work on your machine, it is common to have different config for different environments. For example, you'll probably have different connection URLs for dev, test, and prod.

IoC Config provides an easy mechanism to switch config dependent on your environment. If afIocConfig.env is defined, then any config using that value as a prefix will override any config without.

Consider the following:

afIocConfig.env = prod
jdbcUrl         = jdbc:mysql://localhost:3306/fanlocal

test.jdbcUrl    = jdbc:mysql://localhost:3306/fantest

prod.jdbcUrl    = jdbc:mysql://megacorp:3306/winning

While jdbcUrl would normally have the value jdbc:mysql://localhost:3306/fanlocal, because afIocConfig.env has a value of prod, jdbcUrl is overridden with the value of prod.jdbcUrl. The above could be re-written as:

afIocConfig.env = prod
jdbcUrl         = jdbc:mysql://megacorp:3306/winning

A strategy for defining environmental sensitive config may be to list all possible values, complete with environmental prefixes, in a config.props file. Then switch between the config using a environment variable.

Note that IoC Env automatically sets the afIocConfig.env config value for you according to the ENV environment variable.